Hydrogen sulfide gas is deadly even at low concentrations. Generally, when one is exposed to hydrogen sulfide gas, it is imperative to seek medical attention relatively quickly. Accordingly, in many industrial situations, it is very important to be able to detect the gas in very low concentrations as soon as possible when a leak occurs, even in the most challenging and remote conditions.
Health and safety standards in many countries have been slowly decreasing the acceptable exposure levels as sensor response times and overall stability of sensing elements has improved. For example, in the United States, the Occupational Safety and Health Administration (OSHA) provides an acceptable concentration limit for exposure to hydrogen sulfide at 20 parts per million (ppm) for an 8-hour period, with the maximum peak exposure at 50 ppm for 10 minutes. The UK Health and Safety Executive specifies the maximum acceptable concentration for an 8 hour period at 5 ppm, with the maximum peak exposure for an 8 hour period at 10 ppm. A short-term exposure to even 500-1000 ppm can be life threatening and can cause serious harm. Higher concentrations can cause instant death. There is also evidence that repeated exposure to hydrogen sulfide gas in low concentrations can cause a variety of undesirable medical conditions including photophobia, conjunctivitis, corneal bullae, extreme pain and temporary loss of vision.
An important goal of any fixed-location hydrogen sulfide detector is to safeguard workers and the public by warning of the presence of hazardous levels of hydrogen sulfide in the proximity. Electrochemical and metal oxide semiconductor (MOS) cells have, for many years, been field-proven toxic sensing technologies. Metal oxide semiconductors have a long life compared to electrochemical sensors and continue to operate in wide ranging temperatures, particularly high temperatures, as well as in extremely dry conditions.
In some implementations, a hydrogen sulfide sensor is constructed as a sandwich of a platinum heater element, an insulation medium and the gas sensitive resistive film. In other implementations, a hydrogen sulfide sensor is constructed as a bead having a heater disposed therein and a leadwire running through the bead. The bead is formed of a gas-sensitive semiconductor. This gas sensitive material will employ traditional metal oxide semiconductor materials or metal oxide semiconductor materials that are enhanced at the nano-level to dramatically improve performance. During operation, when hydrogen sulfide gas comes into contact with the gas sensitive material, there is a measurable change in the electrical conductivity. These changes are typically amplified using electronics in a detector device.
The recent advances in nano-enhanced material construction have been able to effectively deal with some of the challenges that limited traditional metal oxide semiconductors. While the appearance and operating principle of a nano-enhanced metal oxide semi-conductor (NE-MOS) is identical to that of a traditional MOS sensor, NE-MOS benefits from a mechanically conformed array of sensing components known as “nanotubes” being applied to the resistive film in a manner in which they are perfectly aligned, symmetric, and extremely concentrated during the manufacturing process. Traditional MOS materials are produced using a process that leaves gaps and creates irregularities, resulting in performance challenges. Nano-enhanced materials provide increased overall sensing capability, faster response, and much higher stability.
As the technology of hydrogen sulfide gas sensors improves and sensing becomes more precise, changes in response of the sensor to hydrogen sulfide caused by changes in environmental conditions become dominant in determining the precision of the sensor. Providing a hydrogen sulfide gas detector that is better able to operate in a wide variety of environments represents an important advance in sensing hydrogen sulfide gas.